Impact-modified polymers, e.g., polymers admixed or copolymerized with a rubber, have a balance between ductility (impact resistance) and stiffness (modulus). The known method to improve ductility is to increase the rubber content of the modified polymer. However, the higher the rubber content in the bulk polymer, the less stiff the polymer. In applications in which a certain level of stiffness is required, an increase of rubber content can require an increase in wall thickness (to maintain a certain level of deflection under load). This, in turn, means more material to produce the article and this, in turn, means more cost to produce the article.
One alternative method of optimizing the ductility/stiffness balance of a rubber-modified polymer is to decrease the average size of the rubber particles. However, this is not desirable for certain applications, e.g., for rubber-modified polymers intended for use in high environmental stress crack resistance (ESCR) applications where large rubber particles are required.
Of interest to the manufacturers of rubber-modified monovinylidene aromatic polymers, is a rubber-modified polymer in which the ductility/stiffness balance is improved (see FIG. 3) without increasing the amount of rubber in the polymer and/or decreasing the average size of the rubber particle.